The present invention relates to a filtration membrane and a method of making this membrane. More specifically, this invention relates to a hollow fiber membrane for use in filtering liquids to remove very large soluble molecules and suspended particles in the liquid.
Membranes are thin-film barriers that allow certain components of a fluid mixture to selectively pass through the membrane while discriminating against other components in the mixture to achieve separation. Membranes are typically formed from polymers and are semi-permeable. The specific physical shape or form of a membrane can vary and can include flat sheets, tubular membranes, and hollow fibers. The specific use to which a membrane is to be put dictates the form selected for its construction. Membranes in the form of hollow fibers are currently used in a variety of applications, including dialysis, gas separation, and ultrafiltration.
A microfiltration membrane filters out suspended particles from a fluid mixture. Normally, microfiltration membranes are made as flat sheets. One disadvantage of flat sheet membranes is that they require more volumetric space than a hollow fiber membrane to provide the same filtration surface area.
Unfortunately, it is difficult to make a hollow fiber microfiltration membrane. The formulation and casting conditions used to obtain a flat sheet microfiltration membrane normally are not applicable for forming a hollow fiber membrane because the viscosity of the casting solution is too low and gravity causes the extruded solution to be broken into pieces.
To obtain a hollow fiber membrane using the casting conditions for a flat sheet microfiltration membrane, a casting solution with a much higher polymer concentration and viscosity is needed. The membrane obtained under such conditions is usually an ultrafiltration membrane rather than a microfiltration membrane. The resulting ultrafiltration membrane not only filters suspended particles out of a liquid mixture but also filters macromolecules out of the liquid mixture. For this reason, the ultrafiltration membrane, unlike a microfiltration membrane, cannot be used only to filter suspended particles from wine, juice, other beverages, and pharmaceutical compounds.
In order to obtain a hollow fiber microfiltration membrane, different formulations and casting conditions from those used to make a flat sheet microfiltration membrane must be employed. U.S. Pat. No. 4,612,119 discloses a method of making a hollow fiber microfiltration membrane that employs a casting solution made from polysulfone, propylene glycol and N-methyl-pyrrolidone. The inside coagulating liquid includes either 70% N-methyl-pyrrolidone aqueous solution, a liquid mixture of methanol and acetone at 1:1 weight ratio, or water. Under these conditions, the hollow fiber membranes obtained are micropourous, but have a very broad pore size distribution. As such, the broad pore size distribution of the membrane limits its applications due to its poor selectivity.
U.S. Pat. No. 5,151,227 discloses another method for making a hollow fiber microfiltration membrane that involves using a continuous spinning process and a casting solution that includes polysulfone, PVP and N-methyl-pyrrolidone. The membrane solution and the bore fluid are delivered by hydraulic pressure rather than a metering pump. However, delivery of bore fluid and membrane solution by hydraulic pressure decreases the precision of the delivery of the fluids. Under these conditions, the hollow fiber membranes obtained also have a broad pore size distribution limiting their applications.
U.S. Pat. No. 4,399,035 discloses yet another method of making a hollow fiber microfiltration membrane. This method involves using a casting solution that includes polyvinylidene fluoride, dimethylacetamide and one or two surfactants, such as polypropylene glycol or polyoxyethylene sorbitan monooleate. The casting solution is extruded into a hollow fiber by means of a gear pump from an annular hollow filament spinning nozzle. One disadvantage with this method is that the large amount of surfactant used in the casting solution not only slows down the phase inversion rate and productivity but also produces a lot of foam that is difficult to handle on a large production scale.
In order to overcome the disadvantages of membranes currently available, a hollow fiber microfiltration membrane with a narrow pore size distribution is needed. Still further, a process for making such a membrane that is efficient and is easily reproducible is needed.